The 24-carat fake

From a solemn TV studio in New York, a news presenter is interviewing Rudolph Giuliani, the city's former mayor. Giuliani, a Republican, is commenting on the first presidential debate of 2004 and criticising John Kerry's wavering stance on the war. 'I agree with you,' says the presenter politely, 'he's had trouble articulating that in the past, but tonight he seemed relatively consistent.'

'Really?' shouts Giuliani, leading the presenter to smirk in faux-submission: 'Coulda been my TV. Might well have been my TV.' The studio audience dissolves into laughter and Giuliani is doomed. The presenter is The Daily Show 's Jon Stewart, a man advertised on billboards as 'the most trusted name in fake news'. The phrase is a spoof of CNN's motto and Stewart's show, which airs at 11pm on Monday to Thursday nights, has made him a cult hero.

So much so that, in some respects, the news he brings is not fake at all. Part news bulletin, part talk-show, The Daily Show features a set of 'fake' correspondents and a series of celebrity guests. But The Daily Show , now in its sixth year, is considered to be so influential that its correspondents are actually invited to political party conventions. No longer entirely artificial, they wield far more clout as a new breed of reporter: comedians with press passes. And the celebrity guests are real, too: both Bill and Hillary Clinton have been on, as has John Kerry. Once you get to that level, what exactly is fake?

A survey conducted by the Pew Research Centre earlier this year showed that 21 per cent of 18- to 29-year-olds got their regular campaign news from The Daily Show and Saturday Night Live . The show has won five Emmys (two of them this year) - and, more unusually for a comedy programme, a Peabody Award for its reporting of the 2000 presidential campaign. It has been argued - by politicians themselves as well as pundits - that an appearance on The Daily Show will do a politician more good among a younger crowd than, say, an appearance on Meet the Press. Research has shown The Daily Show audiences to be highly informed, and General Wesley Clark, a former guest, has said Jon Stewart has 'tremendous influence'.

Stewart, however, has staunchly defended his phony position. 'Our politics are fuelled by comedy,' he has said. 'We are not a power base in any way.' When John Edwards announced on the show that he would be running for President, Stewart's response was: 'I have to warn you we are a fake show, so you might have to do this again somewhere.'

Stewart was born Jonathan Stuart Leibowitz, and brought up in New Jersey. His father is a physicist and his mother taught gifted children. He grew up fascinated by politics, but disdainful, in his own description, of politicians themselves. After college, he moved to New York and changed his name once on the comedy circuit, to avoid the kind of racial taunting he'd received at school. In the early 1990s, he had his own show on MTV, and then spent time writing and performing on The Larry Sanders Show . Since taking over at The Daily Show in 1999, he has changed the format, so that it is a sharper, more serious satire on politics and the media, and tripled the viewing figures. The show, which is on cable, now has an audience of more than a million.

Part of the show's success is Stewart's intelligent and low-key screen persona. He has a trademark raised-eyebrow look of scepticism, but his most striking feature is that of being a softly spoken, all-round nice guy. Ben Karlin, who was hired by Stewart as head writer in 1999 and is now executive producer of the show, explains that although Stewart is 'the main force on the show', he 'realises that not having to be the subject of the story himself actually strengthens everything. Those other late-night shows ... I think Conan O'Brien's show is excellent, but it is driven by Conan's personality.

'Most comedy,' Karlin goes on, 'aims to distract - it's not so intently issue-driven, and when it is issue-driven, it gets didactic and shrill. And Jon, because he's such a performer, is able to deliver material that's actually quite substantive and still keep it genuinely funny. And I can't think of anyone else who's able to do that.'

Stewart, Karlin and the other writers of The Daily Show have just produced a book, which has instantly shot to number one in the New York Times bestseller list. America (The Book): A Citizen's Guide to Democracy Inaction is designed like a school textbook. One 'Fun Task' is an empty outline of the Middle East, with the instructions: 'What do you think the borders of the Middle East should be? Don't be afraid to group people with no regard for history and ethnicity. It worked for the British and the French!' A guide to Europe lists 'famous Europeans: Ernest Hemingway, Gertrude Stein, Johnny Depp and Jim Morrison'.

Karlin is proud of the book, but, like Stewart, he will not admit their team has any public influence. 'There aren't scientific machines sensitive enough to detect how negligible our effect is,' he says. Stewart is contracted to stay on at The Daily Show through the 2008 election. We'll see how wrong they turn out to be.